Motor grader and clutch-control method for motor grader

CROSS-REFERENCE TO RELATED APPLICATIONS

This national phase application claims priority to Japanese Patent Application No. 2007-087547, filed on Mar. 29, 2007. The entire disclosure of Japanese Patent Application No. 2007-087547 is hereby incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a motor grader and a clutch-control method for the motor grader.

BACKGROUND ART

Motor graders are conventionally well-known as a type of construction vehicle for preparing smooth road or ground surfaces. The motor grader is provided with an engine, undercarriage wheels that are rotatably driven by driving force from the engine, and a driving force-transmitting mechanism for transmitting the driving force from the engine to the wheels. Gears can be changed by changing the hydraulic clutch provided to the driving force-transmitting mechanism. The motor grader has a blade that is driven by a hydraulic cylinder. The motor grader moves in a state in which the blade contacts the road surface, whereby a variety of operations such as those given above can be performed (see Japanese Laid-Open Patent Application No. 2003-343325).

In dump trucks, however, a hydraulic-pressure waveform for controlling the clutch is generally modified according to changes in running resistance (see Japanese Laid-Open Patent Application No. 2002-295664). The ratio of speed of the torque converter in the dump truck is used to determine a large, intermediate, or small level of running resistance, and the clutch is controlled using a waveform corresponding to each of the levels. Clutch control can thereby be performed according to the driving conditions, e.g., running on highly uneven construction sites, soft ground, or the like, and shock can be suppressed by changing the clutch.

SUMMARY OF THE INVENTION

When performing ground-leveling operations using a blade, the motor grader described above moves in a state in which the blade contacts the ground. General road travel may also be performed in a state in which the blade is raised so as to avoid contact with the ground. However, the aforedescribed dump-truck-type clutch control does not allow differentiation between the state in which the vehicle performs ground-leveling operations using the blade and the state in which travel occurs without using the blade. Suppressing shock upon switching the clutch is therefore difficult when performing clutch control in a motor grader as described above.

It is one object of the present invention to provide a motor grader that allows shocks to be suppressed upon switching the clutch.

A motor grader according to a first aspect of the present invention includes an engine, a wheel, a driving force-transmitting mechanism, a working equipment cylinder, a blade, a cylinder-hydraulic-pressure-detecting section, and a control section. The wheel is configured and arranged to be rotatably driven by a driving force from the engine. The driving force-transmitting mechanism is configured and arranged to transmit the driving force from the engine to the wheel and has a clutch that is configured and arranged to e driven by hydraulic pressure. The working equipment cylinder is a hydraulic cylinder configured and arranged to be driven by hydraulic pressure. The blade is configured and arranged to be driven by the working equipment cylinder. The cylinder-hydraulic-pressure-detecting section is configured and arranged to detect the hydraulic pressure supplied to the working equipment cylinder. The control section is configured to determine that the motor grader is in a running state when the cylinder hydraulic pressure detected by the cylinder-hydraulic-pressure-detecting section is less than a predetermined threshold value and to use a running-hydraulic-pressure waveform corresponding to the running state to control the hydraulic pressure supplied to the clutch. The control section is configured to determine that the motor grader is in a working state when the cylinder hydraulic pressure is greater than or equal to the threshold value and to use a working-hydraulic-pressure waveform corresponding to the working state to control the hydraulic pressure supplied to the clutch.

In this motor grader, a judgment is made as to whether or not the hydraulic pressure of the working equipment cylinder that drives the blade is less than a predetermined threshold value. The load on the working equipment cylinder is small when the vehicle is in a running state, and therefore the cylinder hydraulic pressure decreases. The load on the working equipment cylinder is large when the vehicle is in a working state, and therefore the cylinder hydraulic pressure increases. Therefore, by comparing the hydraulic pressure of the working equipment cylinder to a predetermined threshold value, a judgment can be accurately made as to whether the vehicle is in the running state or the working state. A running-hydraulic-pressure waveform corresponding to the running state is used to control the hydraulic pressure supplied to the clutch when a judgment is made that the vehicle is in the running state. A working-hydraulic-pressure waveform corresponding to the working state is used to control the hydraulic pressure supplied to the clutch when a judgment is made that the vehicle is in the working state. As a result, appropriate clutch control can be performed in this motor grader during running and working, and shock can be suppressed when switching the clutch.

Depending on the magnitude of the threshold value, cases in which the state is judged to be the running state also include cases of running while the blade is in contact with the ground and subjected to a relatively light load.

A motor grader according to a second aspect of the present invention is the motor grader according to the first aspect, wherein the working equipment cylinder is a lift cylinder configured and arranged to move the blade in a vertical direction.

In this motor grader, the cylinder hydraulic pressure of a lift cylinder for causing the blade to move in the vertical direction is compared with the predetermined threshold value. The lift cylinder maintains the blade in a state pressed to the ground in the working state and maintains the blade in a raised state in the running state. The cylinder hydraulic pressure of the lift cylinder is therefore different between the running state and the working state. The cylinder hydraulic pressure also changes between performing operations under a large load and performing operations under a small load. The control section can therefore use the cylinder hydraulic pressure of the lift cylinder to accurately judge the state of the vehicle.

A motor grader according to a third aspect of the present invention is the motor grader according to the first aspect, wherein the working equipment cylinder is a tilt cylinder configured and arranged to change an angle of the blade.

In this motor grader, the cylinder hydraulic pressure of a tilt cylinder for changing the angle of the blade is compared with the threshold value. The load on the blade from the ground changes depending on the angle of the blade. The cylinder hydraulic pressure of the tilt cylinder changes when the angle of the blade changes. The control section can therefore accurately judge the state of the vehicle using the cylinder hydraulic pressure of the tilt cylinder.

A clutch-control method for a motor grader of a fourth aspect of the present invention is a method for controlling a hydraulic clutch provided to a motor grader. The method includes a step for detecting cylinder hydraulic pressure, a determination step, a running-state clutch control step, and a working-state clutch control step. In the step for detecting cylinder hydraulic pressure, a hydraulic pressure supplied to a working equipment cylinder for driving a blade is detected. In the determination step, a determination is made as to whether or not the cylinder hydraulic pressure detected in the step for detecting cylinder hydraulic pressure is less than a predetermined threshold value. In the running-state clutch control step, a determination is made that the motor grader is in a running state when the cylinder hydraulic pressure is less than the threshold value, and a running-hydraulic-pressure waveform corresponding to the running state is used to control the hydraulic pressure supplied to the clutch. In the working-state clutch control step, a determination is made that the motor grader is in a working state when the cylinder hydraulic pressure is greater than or equal to the predetermined threshold value, and a working-hydraulic-pressure waveform corresponding to the working state is used to control the hydraulic pressure supplied to the clutch.

In this clutch-control method for a motor grader, a determination is made as to whether or not the hydraulic pressure of the working equipment cylinder that drives the blade is less than a predetermined threshold value. The load on the working equipment cylinder is small when the vehicle is in a running state, and therefore the cylinder hydraulic pressure decreases. The load on the working equipment cylinder is large when the vehicle is in a working state, and therefore the cylinder hydraulic pressure increases. Therefore, by comparing the hydraulic pressure of the working equipment cylinder to a predetermined threshold value, a determination can be accurately made as to whether the vehicle is in the running state or the working state. A running-hydraulic-pressure waveform corresponding to the running state is used to control the hydraulic pressure supplied to the clutch when a determination is made that the vehicle is in the running state. A working-hydraulic-pressure waveform corresponding to the working state is used to control the hydraulic pressure supplied to the clutch when a determination is made that the vehicle is in the working state. As a result, appropriate clutch control can be performed in this motor grader during running and working, and shock can be suppressed when switching the clutch.

BRIEF DESCRIPTION OF THE DRAWINGS

<figref idrefs="DRAWINGS">FIG. 1</figref> is an external perspective view of the motor grader;

<figref idrefs="DRAWINGS">FIG. 2</figref> is a lateral view of the motor grader;

<figref idrefs="DRAWINGS">FIG. 3</figref> is a schematic diagram of the configuration of the driving force-transmitting mechanism, the hydraulic driving mechanism, and the control section;

<figref idrefs="DRAWINGS">FIG. 4</figref> is a flow chart that shows the clutch control;

<figref idrefs="DRAWINGS">FIG. 5</figref> includes timing charts (a) and (b) that show the hydraulic-pressure waveforms of the clutch control;

<figref idrefs="DRAWINGS">FIG. 6</figref> is a flow chart that shows the clutch control according to another embodiment; and

<figref idrefs="DRAWINGS">FIG. 7</figref> is a flow chart that shows the clutch control according to another embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Configuration

Overall Configuration

An external perspective view and a lateral view of a motor grader <b>1</b> according to an embodiment of the present invention are shown in <figref idrefs="DRAWINGS">FIGS. 1 and 2</figref>. The motor grader <b>1</b> is provided with six wheels composed of a pair of left and right front wheels <b>11</b> and rear wheels <b>12</b> (two on each side). A blade <b>42</b> provided between the front wheels <b>11</b> and the rear wheels <b>12</b> can be used for ground leveling, snow removal, light cutting, mixing materials, and other operations. Of the four rear wheels <b>12</b>, only the wheels positioned on the left side are shown in <figref idrefs="DRAWINGS">FIGS. 1 and 2</figref>.

The motor grader <b>1</b> is provided with a frame <b>2</b>, a cabin <b>3</b>, and a working equipment <b>4</b>, as shown in <figref idrefs="DRAWINGS">FIGS. 1 and 2</figref>. The motor grader <b>1</b> is also provided with an engine <b>5</b>, a driving force-transmitting mechanism <b>6</b>, a hydraulic drive mechanism <b>7</b>, a control section <b>8</b>, and the like, as shown in <figref idrefs="DRAWINGS">FIG. 3</figref>.

Frame

2

and Cabin

3

The frame <b>2</b> comprises a rearward frame <b>21</b> and a forward frame <b>22</b>, as shown in <figref idrefs="DRAWINGS">FIGS. 1 and 2</figref>.

The rearward frame <b>21</b> houses the engine <b>5</b>, the driving force-transmitting mechanism <b>6</b>, the hydraulic drive mechanism <b>7</b>, and the like shown in <figref idrefs="DRAWINGS">FIG. 3</figref>. The four rear wheels <b>12</b> are provided to the rearward frame <b>21</b>. The rear wheels <b>12</b> are rotationally driven by the driving force from the engine <b>5</b>, whereby the vehicle can travel.

The forward frame <b>22</b> is attached to the front of the rearward frame <b>21</b>. The front wheels <b>11</b> are attached to the front end of the forward frame <b>22</b>.

The cabin <b>3</b> is mounted on the rearward frame <b>21</b>. Handles, a gear-change lever, levers for operating the working equipment <b>4</b>, other components of an operation section <b>13</b> (see <figref idrefs="DRAWINGS">FIG. 3</figref>), in accelerator <b>14</b>, a brake, an inching pedal (not shown), and the like are provided to the interior of the cabin. The cabin <b>3</b> may also be mounted on the forward frame <b>22</b>.

Working Equipment

4

The working equipment <b>4</b> has, e.g., a drawbar <b>40</b>, a circle <b>41</b>, the blade <b>42</b>, a hydraulic motor <b>49</b>, hydraulic cylinders <b>44</b> through <b>48</b>, and the like.

The front end of the drawbar <b>40</b> is swivelably attached to the front end of the forward frame <b>22</b>. The rear end of the drawbar <b>40</b> rises and falls due to synchronous expansion and contraction of the pair of lift cylinders <b>44</b>, <b>45</b>. The drawbar <b>40</b> is tilted in the vertical direction due to differential expansion and contraction of the lift cylinders <b>44</b>, <b>45</b>. The drawbar <b>40</b> also swivels up and down about the axis along the direction of travel of the vehicle due to expansion and contraction of the drawbar shift cylinder <b>46</b>.

The circle <b>41</b> is rotatably attached to the rear end of the drawbar <b>40</b>. The circle <b>41</b> is driven by the hydraulic motor <b>49</b> (see <figref idrefs="DRAWINGS">FIG. 1</figref>) and rotates in a clockwise or counter-clockwise direction relative to the drawbar <b>40</b> as viewed from above the vehicle.

The blade <b>42</b> is supported so as to be able to slide in the horizontal direction relative to the circle <b>41</b> and so as to be able to swivel up and down about an axis parallel to the horizontal direction. By “horizontal direction” is meant the lateral direction relative to the direction of travel of the vehicle. The blade shift cylinder <b>47</b>, which is supported by the circle <b>41</b>, allows the blade <b>42</b> to move in the horizontal direction relative to the circle <b>41</b>. The tilt cylinder <b>48</b> (see <figref idrefs="DRAWINGS">FIG. 2</figref>) supported by the circle <b>41</b> allows the blade <b>42</b> to swivel about the axis parallel to the horizontal direction and change the vertical orientation relative to the circle <b>41</b>. The blade <b>42</b> as described above can, via the drawbar <b>40</b> and the circle <b>41</b>, rise or fall vertically with respect to the vehicle, change in tilt orientation relative to the direction of travel, change in tilt orientation relative to the horizontal direction, rotate, and shift in the lateral direction.

The hydraulic motor <b>49</b> is driven by hydraulic fluid supplied from a first hydraulic pump <b>71</b> (described hereinafter), whereby the circle <b>41</b> is made to rotate.

The various hydraulic cylinders <b>44</b> through <b>48</b> are driven by hydraulic pressure provided from the first hydraulic pump <b>71</b>. These cylinders include, e.g., the pair of lift cylinders <b>44</b>, <b>45</b>, the drawbar shift cylinder <b>46</b>, the blade shift cylinder <b>47</b>, the tilt cylinder <b>48</b>, and the like as described above. The pair of lift cylinders <b>44</b>, <b>45</b> are provided separated on the left and right sides sandwiching the forward frame <b>22</b> between them. The lift cylinders <b>44</b>, <b>45</b> are positioned substantially along the vertical direction and are attached to the forward frame <b>22</b> and the drawbar <b>40</b>. The expansion and contraction of the lift cylinders <b>44</b>, <b>45</b> causes the rear end of the drawbar <b>40</b> to rise and fall, whereby the blade <b>42</b> can be made to move in the vertical direction. The drawbar shift cylinder <b>46</b> is positioned slantwise relative to the vertical direction and is attached to the lateral end of the forward frame <b>22</b> and the drawbar <b>40</b>. The expansion and contraction of the drawbar shift cylinder <b>46</b> allows the slant angle of the drawbar <b>40</b> relative to the horizontal direction to be changed, whereby the slant angle of the blade <b>42</b> can be changed. The blade shift cylinder <b>47</b> is positioned along the longitudinal direction of the blade <b>42</b> and is attached to the circle <b>41</b> and the blade <b>42</b>. The expansion and contraction of the blade shift cylinder <b>47</b> allows the longitudinal position of the blade <b>42</b> to be changed. The tilt cylinder <b>48</b> is attached to the circle <b>41</b> and the blade <b>42</b>. The expansion and contraction of the tilt cylinder <b>48</b> allows the blade <b>42</b> to swivel up and down about the axis along the horizontal direction, whereby the tilt angle of the blade <b>42</b> relative to the direction of travel can be changed.

Engine

5

As shown in <figref idrefs="DRAWINGS">FIG. 3</figref>, a fuel injection pump <b>15</b> is affixed to the engine <b>5</b>. Fuel is provided to the engine <b>5</b> from the fuel injection pump <b>15</b>. The amount of fuel is controlled by an injection-volume signal output from the control section <b>8</b> (described hereinafter) to an electronic governor <b>16</b>. The control section <b>8</b> controls the amount of fuel supplied to the engine <b>5</b>, whereby the rotational speed of the engine <b>5</b> can be controlled.

Driving Force-Transmitting Mechanism

6

The driving force-transmitting mechanism <b>6</b> is designed to transmit driving force from the engine <b>5</b> to the rear wheels <b>12</b>. The driving force-transmitting mechanism <b>6</b> has a transmission <b>60</b> and a tandem device <b>61</b>.

The transmission <b>60</b> has, e.g., a torque convertor <b>62</b>, various clutches <b>63</b> through <b>69</b>, and a plurality of gears used to change speed (not shown).

The torque convertor <b>62</b> is connected to the output side of the engine <b>5</b>. A lock-up clutch <b>70</b> for joining the input shaft and the output shaft of the torque convertor <b>62</b> is provided to the torque convertor <b>62</b>. A switch included in the aforedescribed operation section <b>13</b> is operated, whereby the lock-up clutch <b>70</b> switches between an engaged state and a disengaged state. Driving force is transmitted from the engine <b>5</b> without passing through the torque convertor <b>62</b> when the lock-up clutch <b>70</b> is in an engaged state. Driving force from the engine <b>5</b> is transmitted via the torque convertor <b>62</b> when the lock-up clutch <b>70</b> is in a disengaged state.

The various clutches <b>63</b> through <b>69</b> are hydraulic clutches that are driven by hydraulic pressure supplied from a second hydraulic pump <b>72</b> (described hereinafter). These clutches include the FL clutch <b>63</b>, the FH clutch <b>64</b>, the R clutch <b>65</b>, the first clutch <b>66</b>, the second clutch <b>67</b>, the third clutch <b>68</b>, and the fourth clutch <b>69</b>. The FL clutch <b>63</b> and the FH clutch <b>64</b> are engaged during forward travel of the vehicle. The R clutch <b>65</b> is engaged during backward travel of the vehicle. The first clutch <b>66</b>, the second clutch <b>67</b>, the third clutch <b>68</b>, and the fourth clutch <b>69</b> are engaged during transmission of driving force to the corresponding transmission gears. During forward travel, speed levels one through eight can be selected in the transmission <b>60</b> using a combination of either the FL clutch <b>63</b> or the FH clutch <b>64</b> and one of the first clutch <b>66</b> through the fourth clutch <b>69</b>. During backward travel, speed levels one through four can be selected using a combination of the R clutch <b>65</b> and one of the first clutch <b>66</b> through the fourth clutch <b>69</b>.

The driving force output from the transmission <b>60</b> is transmitted to the rear wheels <b>12</b> via a final reduction gear (not shown) and the tandem device <b>61</b>.

Hydraulic Drive Mechanism

7

The hydraulic drive mechanism <b>7</b> is a mechanism for producing hydraulic pressure using the driving force from the engine <b>5</b> and driving the various clutches <b>63</b> through <b>69</b>, the hydraulic motor <b>49</b>, and the various cylinders <b>44</b> through <b>48</b> described above. The hydraulic drive mechanism <b>7</b> comprises the first hydraulic pump <b>71</b>, the second hydraulic pump <b>72</b>, various hydraulic-pressure control valves <b>73</b> through <b>78</b>, <b>51</b> through <b>57</b>, and various sensors <b>83</b> through <b>88</b>.

The first hydraulic pump <b>71</b> is driven by driving force from the engine <b>5</b> and produces hydraulic pressure supplied to the various cylinders <b>44</b> through <b>48</b> and the hydraulic motor <b>49</b>. The first hydraulic pump <b>71</b> is a variable-displacement hydraulic pump that uses a pump-volume control cylinder <b>71</b> a to change the tilt angle of a swashplate, thereby changing the volume of hydraulic fluid discharged.

The second hydraulic pump <b>72</b> is driven by driving force from the engine <b>5</b> and produces hydraulic pressure supplied to the various clutches <b>63</b> through <b>69</b>.

The various hydraulic-pressure control valves <b>73</b> through <b>78</b>, <b>51</b> through <b>57</b> are electromagnetic proportional control valves that are electrically controlled by the control section <b>8</b>, whereby hydraulic pressure can be regulated. These valves include, e.g., the first through fifth cylinder control valves <b>73</b> through <b>77</b>, the hydraulic-motor control valve <b>78</b>, and the first through seventh clutch control valves <b>51</b> through <b>57</b>.

The first through fifth cylinder control valves <b>73</b> through <b>77</b> regulate the hydraulic pressure supplied to the various cylinders <b>44</b> through <b>48</b> described above. Specifically, the first cylinder control valve <b>73</b> regulates the hydraulic pressure supplied to the lift cylinder <b>44</b>. The second cylinder control valve <b>74</b> regulates the hydraulic pressure supplied to the lift cylinder <b>45</b>. The third cylinder control valve <b>75</b> regulates the hydraulic pressure supplied to the drawbar shift cylinder <b>46</b>. The fourth cylinder control valve <b>76</b> regulates the hydraulic pressure supplied to the blade shift cylinder <b>47</b>. The fifth cylinder control valve <b>77</b> regulates the hydraulic pressure supplied to the tilt cylinder <b>48</b>.

The hydraulic-motor control valve <b>78</b> regulates the hydraulic pressure supplied to the hydraulic motor <b>49</b> described above.

The first through seventh clutch control valves <b>51</b> through <b>57</b> regulate the hydraulic pressure supplied to the various clutches <b>63</b> through <b>69</b> described above. Specifically, the first clutch control valve <b>51</b> regulates the hydraulic pressure supplied to the FL clutch <b>63</b>. The second clutch control valve <b>52</b> regulates the hydraulic pressure supplied to the FH clutch <b>64</b>. The third clutch control valve <b>53</b> regulates the hydraulic pressure supplied to the R clutch <b>65</b>. The fourth clutch control valve <b>54</b> regulates the hydraulic pressure supplied to the first clutch <b>66</b>. The fifth clutch control valve <b>55</b> regulates the hydraulic pressure supplied to the second clutch <b>67</b>. The sixth clutch control valve <b>56</b> regulates the hydraulic pressure supplied to the third clutch <b>68</b>. The seventh clutch control valve <b>57</b> regulates the hydraulic pressure supplied to the fourth clutch <b>69</b>.

The various sensors <b>83</b> through <b>88</b> detect the hydraulic pressure supplied to various hydraulic actuators <b>44</b> through <b>49</b> via the aforedescribed hydraulic-pressure control valves <b>73</b> through <b>78</b> and send a detection signal to the control section <b>8</b>. The detection signal corresponds to the magnitude of the detected hydraulic pressure. Specifically, the first hydraulic-pressure sensor <b>83</b> detects the hydraulic pressure supplied to the lift cylinder <b>44</b>. The second hydraulic-pressure sensor <b>84</b> detects the hydraulic pressure supplied to the lift cylinder <b>45</b>. The third hydraulic-pressure sensor <b>85</b> detects the hydraulic pressure supplied to the drawbar shift cylinder <b>46</b>. The fourth hydraulic-pressure sensor <b>86</b> detects the hydraulic pressure supplied to the blade shift cylinder <b>47</b>. The fifth hydraulic-pressure sensor <b>87</b> detects the hydraulic pressure supplied to the tilt cylinder <b>48</b>. The sixth hydraulic-pressure sensor <b>88</b> detects the hydraulic pressure supplied to the hydraulic motor <b>49</b>.

Control Section

8

The control section <b>8</b> controls the first through fifth cylinder control valves <b>73</b> through <b>77</b> and the hydraulic-motor control valve <b>78</b> on the basis of, e.g., command signals from the operation section <b>13</b> to the working equipment <b>4</b> and detection signals from the various sensors <b>83</b> through <b>88</b>, whereby the position and orientation of the blade <b>42</b> can be changed. The control section <b>8</b> also controls the first through seventh clutch control valves <b>51</b> through <b>57</b> on the basis of, e.g., command signals from the accelerator <b>14</b>, the gear-change lever, and the like, and detection signals from other hydraulic-pressure sensors (not shown), whereby gear changes can be controlled appropriately according to the state of the vehicle.

Among the gear-change control methods of the control section <b>8</b>, a clutch-control method used during gear changes will be described in detail below.

Clutch-Control Method

A flowchart of clutch control in the control section <b>8</b> is shown in <figref idrefs="DRAWINGS">FIG. 4</figref>.

In a first step S<b>1</b> (step for detecting cylinder hydraulic pressure), the hydraulic pressure supplied to the lift cylinders <b>44</b>, <b>45</b> (referred to as “cylinder hydraulic pressure P<b>1</b>” below) is detected. The larger of the values for hydraulic pressure detected by the first hydraulic-pressure sensor <b>83</b> and the second hydraulic-pressure sensor <b>84</b> is referenced in the control section <b>8</b>. The average value of the hydraulic pressure detected by the first hydraulic-pressure sensor <b>83</b> and the second hydraulic-pressure sensor <b>84</b> may be referenced in the control section <b>8</b>.

In a second step S<b>2</b> (determination step), a determination is made as to whether or not the cylinder hydraulic pressure P<b>1</b> detected in the first step S<b>1</b> is less than a predetermined threshold value A. Flow proceeds to a third step S<b>3</b> when the cylinder hydraulic pressure P<b>1</b> is not less than the predetermined threshold value A, i.e., when the cylinder hydraulic pressure P<b>1</b> is greater than or equal to the predetermined threshold value A. Flow proceeds to a fourth step S<b>4</b> when the cylinder hydraulic pressure P<b>1</b> is less than the predetermined threshold value A.

In the third step S<b>3</b> (working-state clutch control step), the control section <b>8</b> judges that the vehicle is in a working state and controls the hydraulic pressure supplied to the first clutch <b>66</b> through the fourth clutch <b>69</b> using working-hydraulic-pressure waveforms corresponding to the working state. Time charts showing these working-hydraulic-pressure waveforms are shown as solid lines La<b>1</b>, Lb<b>1</b> in <figref idrefs="DRAWINGS">FIGS. 5(</figref><i>a</i>) and <b>5</b>(<i>b</i>). The solid line La<b>1</b> in <figref idrefs="DRAWINGS">FIG. 5(</figref><i>a</i>) represents the engaged-clutch hydraulic pressure, i.e., the pressure of the gear-change clutch on the engagement side. The solid line Lb<b>1</b> in <figref idrefs="DRAWINGS">FIG. 5(</figref><i>b</i>) represents the disengaged-clutch hydraulic pressure, i.e., the pressure of the gear-change clutch on the disengagement side. These time charts show a case in which, among the first clutch <b>66</b> through the fourth clutch <b>69</b>, the currently engaged gear-change clutch (e.g., the first clutch <b>66</b>) is disengaged, and the gear-change clutch (e.g., the second clutch <b>67</b>) corresponding to the selected gear is engaged, whereby a gear change is performed. Before the gear-change clutch on the disengagement side is released, the gear-change clutch on the engagement side is filled with hydraulic fluid. Once filling is completed, the gear-change clutch on the disengagement side is then released, after which the hydraulic pressure of the gear-change clutch on the engagement side is gradually increased. The occurrence of gear-change shocks is thereby suppressed.

In the fourth step S<b>4</b> (running-state clutch control step), the control section <b>8</b> judges that the vehicle is in a running state and controls the hydraulic pressure supplied to the first clutch <b>66</b> through the fourth clutch <b>69</b> using running-hydraulic-pressure waveforms corresponding to the running state. These running-hydraulic-pressure waveforms are set to a lower hydraulic pressure than the working-hydraulic-pressure waveforms, as shown by the broken lines La<b>2</b>, Lb<b>2</b> in <figref idrefs="DRAWINGS">FIGS. 5(</figref><i>a</i>) and <b>5</b>(<i>b</i>).

The working-hydraulic-pressure waveforms and running-hydraulic-pressure waveforms of the engagement-side gear-change clutch described above are prepared in advance and stored in the control section <b>8</b> according to gear-change conditions that are based on driving conditions, the transmission gears, and the like. Hydraulic-pressure waveforms are selected and employed according to the various gear-change conditions during gear-change control.

Characteristics

In this motor grader <b>1</b>, the magnitude of the hydraulic pressure supplied to the lift cylinders <b>44</b>, <b>45</b> is used to make a determination as to whether the vehicle is in a working state or a running state. The hydraulic-pressure waveforms for controlling the first clutch <b>66</b> through the fourth clutch <b>69</b> are switched between the working state and the running state. Appropriate gear-change control can therefore be performed for the working state and the running state, respectively, and gear-change shocks can be suppressed upon switching the clutch.

Other Embodiments

(a) In addition to the aforedescribed clutch-control method, a determination may also be made as to whether or not a throttle opening Da is larger than a predetermined threshold value B, as shown in a fifth step S<b>5</b> in <figref idrefs="DRAWINGS">FIG. 6</figref>. Flow proceeds to the second step S<b>2</b> when the throttle opening Da is larger than the predetermined threshold value B. The second step S<b>2</b> through the fourth step S<b>4</b> are the same as in the aforedescribed embodiment. Flow proceeds to a sixth step S<b>6</b> when the throttle opening Da is not larger than the predetermined threshold value B, i.e., when the throttle opening Da is less than or equal to the threshold value B. In the sixth step S<b>6</b>, a low-throttle hydraulic-pressure waveform corresponding to the case where the throttle opening Da is small is used to control the first clutch <b>66</b> through the fourth clutch <b>69</b>. This low-throttle hydraulic-pressure waveform is different from both the waveforms for running hydraulic pressure and for working hydraulic pressure.

A judgment concerning the throttle opening Da is thus added, whereby appropriate gear-change control can be performed according to the state of the vehicle.

(b) In addition to the clutch-control method of the aforedescribed embodiment or the other embodiment (a), a judgment may also be made as to whether or not a fuel injection volume Qf is greater than or equal to a predetermined threshold value C, as shown in a seventh step S<b>7</b> in <figref idrefs="DRAWINGS">FIG. 7</figref>. Flow proceeds to the second step S<b>2</b> when the fuel injection volume Qf is greater than or equal to the predetermined threshold value C. The second step S<b>2</b> through the sixth step S<b>6</b> are the same as in the aforedescribed embodiments. Flow proceeds to the fourth step S<b>4</b> when the fuel injection volume Qf is not greater than or equal to the predetermined threshold value C, i.e., when the fuel injection volume Qf is less than the threshold value C.

A judgment concerning the fuel injection volume Qf is thus added, whereby appropriate gear-change control can be performed according to the state of the vehicle.

The judgment may also be made on the basis of the magnitude of the engine torque instead of the fuel injection volume Qf.

(c) In the aforedescribed embodiments, the judgment of running state or working state was made on the basis of the hydraulic pressure supplied to the lift cylinders <b>44</b>, <b>45</b>, but the judgment may also be made on the basis of the hydraulic pressure supplied to the tilt cylinder <b>48</b>. Additionally, the judgment may also be made on the basis of both the hydraulic pressure supplied to the lift cylinders <b>44</b>, <b>45</b> and the hydraulic pressure supplied to the tilt cylinder <b>48</b>.

The motor grader of the described embodiments can suppress shock when switching a clutch.